Hybrid vehicles are typically powered by the combination of a combustion engine and an electric motor. In a “through-the-road” hybrid vehicle, at least two axles are separately driven; the combustion engine is configured to power one axle, while the electric motor is configured to separately and independently power the second axle.
FIG. 1 is a schematic illustration of an exemplary “through-the-road” hybrid vehicle 10, as already known in the art. The vehicle 10 includes a first axle 12, adapted to drive a first pair of wheels 14, 16 through a first differential 18. The vehicle 10 includes a second axle 20, adapted to drive a second pair of wheels 22, 24 through a second differential 26. Although the first pair of wheels 14, 16 as depicted herein may be front wheels, and the second pair of wheels 22, 24 may be rear wheels, either pair of wheels, depending on the nature of a given vehicle 10, may constitute front wheels or rear wheels of the vehicle 10.
The vehicle 10 includes a front axle powertrain 28, including an internal combustion engine 30 coupled with a transmission (or gearbox) 32 through a friction clutch 34. For this purpose the transmission 32 includes an input (not shown) adapted to engage and disengage with the friction clutch 34 at an end of the transmission nearest the engine 30. Internal gearing and an output shaft (neither shown) of the transmission 32 transmit engine torque via the friction clutch 34 from the engine 30 to the first differential 18 by a drive shaft (not shown). In the through-the-road embodiment described herein, the powertrain 28 is configured to only power the first axle 12 and its associated components, including the first differential 18.
As is typical, in the through-the-road vehicle 10 the second, or rear, axle 20 is, unlike the first axle 12, separately and independently powered by an electric motor 40. As such, the electric motor 40 drives the rear wheels 22, 24 through an optional gearbox 42 (to provide, for example, a two-speed gear ratio capability) having an output shaft (not shown) coupled to the second differential 26. Alternatively, the electric motor 40 may have an output shaft coupled directly to the second differential 26 for providing a single speed drive configuration. Irrespective of whether the rear axle is a single or two speed, the electric motor 40, including the second differential 26, and other components configured to drive the second axle 20 may be referred to herein as the rear axle powertrain 44.
Adjunctive to the rear axle powertrain 44 is an inverter 50 configured for selectively providing regenerative power to a rechargeable battery 52 adapted to power the electric motor.
The components of the described hybrid vehicle 10 may be operated and/or controlled in accordance with driving conditions to optimize efficient utilization of the front and rear axle powertrains 28, 44; i.e., the internal combustion engine 30 and the electric motor 40, respectively, in different ways. For example, during a combination of stop and go and/or slower driving in urban areas, the rear electric motor-driven powertrain 44 may be utilized more than the front internal combustion engine powertrain 28 to the extent that the powertrain 44 may offer most efficient power while saving fuel. However, during highway driving with less stop and go and at higher speeds, higher utilization of the power train 28 may prove most efficient, for reasons those skilled in the art may best appreciate.
During use of internal combustion engine powertrain 28, the friction clutch 34 is engaged and disengaged to selectively connect and disconnect the internal combustion engine 30 to the transmission 32 so that power from the engine 30 is delivered to the front axle 12. At the same time the front axle 12 is driven, the electric motor 40 may be controlled to provide additional power boosts to the rear axle 20, or to alternatively use the power supplied by the internal combustion engine 30 to recharge the battery 52. Conversely, during urban driving situations, the friction clutch 34 may be opened to disconnect the engine 30 from driving the front axle 12, and power from the battery 48 may be used by the motor 40 to drive the vehicle 10 via the rear axle 20. During the latter driving condition, the engine 30 may be completely stopped, and/or the friction clutch 34 opened, or disengaged, to conserve fuel. However, during a momentary acceleration of the vehicle 10, the friction clutch 34 may be re-engaged to provide more responsive acceleration, thus utilizing power from both the engine 30 and the motor 40. In another variation, the friction clutch 34 may be opened or disengaged to disconnect the engine 30 during deceleration so that the motor 40 may more efficiently recharge the battery 48; i.e. without power losses due to engine friction.
Under presently known arrangements of the friction clutch 34 in the through-the-road hybrid vehicle 10, the friction clutch 34 in its engaged or closed position is configured to lock the front axle powertrain 28 for rotation of the front axle in either direction. In the disengaged or open position of the friction clutch 34, the front axle 12 is free to rotate in either direction. This arrangement may create inefficiencies in operation of the hybrid vehicle 10. Whenever the friction clutch 34 is engaged for driving the vehicle 10 under the power of the engine 30, or while the vehicle 10 is accelerating under the combined power of the engine 30 and the electric motor 40, any slowing the engine 30 may cause rotating losses as the front axle powertrain 28 slows, unless the friction clutch 34 is actuated to open to disconnect the engine 30 from the transmission 32.
If the friction clutch 34 remains closed, engine rotating losses will be incurred. The latter may be desirable for such vehicles 10 in situations where engine braking is desirable. In most hybrid vehicles, however, regenerative battery power braking is preferred instead, so as to most effectively re-energize the rechargeable battery 52 through the inverter 50. If the friction clutch 34 is actuated to disconnect the engine 30, the engine rotating losses may be avoided, but open friction clutch rotating losses remain, as the relatively large surface areas of the facing clutch plates are subjected to oil shear with resulting viscous drag.
In addition, since the friction clutch 34 must be reclosed whenever the engine 30 is called upon to provide power to the driven wheels 14, 16, the options of leaving the friction clutch 34 closed, may negate efficiencies sought to be achieved by the hybrid vehicle 10. As such, opening and closing the friction clutch 34, may effectively negate the efficiencies sought to be achieved, due to the viscous drag and corresponding open clutch rotating losses, as well as increases in duty cycle required for actuating the friction clutch 34. Therefore, a need exists for an improved strategy for switching between the power sources of a through-the-road hybrid vehicle that can increase energy efficiencies without increasing rotating losses.